Objective To evaluate the genetic contribution to development of endometriosis. Endometriosis is a disease known to affect many primate species including rhesus monkeys and humans. The aim of this study is to investigate the occurrence of spontaneous endometriosis in a colony of rhesus monkeys and to determine whether procedures such a caesarian section, laparoscopy and hormone treatment increase the risk of developing endometriosis. Three hundred eighty-six necropsy reports from female monkeys that died at an age greater than four years were examined. The animals with endometriosis were matched by age at death to animals where no disease was found at death (controls) and the surgical histories were screened. Eighty-one (21%) of the animals had endometriosis and their mean age (qSD) at death was 20.7q5.5 years (range, 10-35 yr). Of these 13 (16%) had minimal/mild disease and 68 (44%) had moderate/severe disease. Endometriosis was also found to have a familial tendency, with 37 (46%) of animals with the disease being related to at least one other affected animal, compared to 22 (27%) that were inter-related in the age-matched control group. The 37 animals with endometriosis that were related fell into 10 distinct family groups. Segregation analysis is being performed to investigate the mode of inheritance of the disease. Key words endometriosis, female reproductive health, genetics of disease